## Brake System

Braking is essential for the safe operation of the HOLI 300 small wind turbine. In this section we explain how to calculate the braking torque. We also find a brake type able to supply this torque.

Important Note

Preliminary content from design report

The content of this article is taken from the December 2013 preliminary design report. It represents intention of design at that stage but does not necessarily show the final version of the HOLI 300 turbine design.

The contest regulations required the wind turbine system to provide a manual blocking of the rotor and a manual emergency stop. In order to fulfill the general safety requirements as described here the turbine had to be protected against overspeed as well. Overspeed can occur by different failures. A precondition for overspeed is high wind speeds. If these occur and the furling system fails to limit the rotor speed as described in the respective article, the disk brake gets activated. Except of a failure in the furling system, overspeed can also be caused by:

• failure of main shaft

The disk brake is triggered by the microcontroller, which determines the overspeed state by means of sensors. In case of overspeed, the brake is configured to stop the turbine in a short amount of time which will be specified in the following paragraph.

### Braking basics

The requirements for the braking system of the wind turbine are principally determined by the necessary braking torque. This torque is calculated from three parts. The necessary torque to break down the inertia of the system, the torque to break down the energy input out of the wind and the frictional resistance. The frictional resistance can be neglected. The inertia of the system can be calculated as follows:

It is assumed that a fault in the safety system or a furling delay leads to a 50 % overspeed condition compared to the cut-out wind speed with a rotor speed of $n=476.94\,\frac{1}{\textrm{min}}\cdot1.5\thickapprox715\,\frac{1}{\textrm{min}}$ and the brake should completely stop the rotor in a time of $t=3\,\textrm{s}$. This assumption is made in addition to IEC 61400-2 Load Case G to fulfill the customer’s high safety requirements. Also, it ensures a safe shutdown if the yaw axis is blocked and as a result, the furling system cannot move the rotor out of the wind. With an assumed blade inertia $J_{\textrm{b}}=0.305\,\textrm{kgm}^{2}$, generator inertia $J_{\textrm{g}}=0.006\,\textrm{kgm}^{2}$ and an estimated inertia taking into consideration other hub components as spinner and shaft $J_{\textrm{o}}=0.006\,\textrm{kgm}^{2}$, the total hub inertia is $J_{\textrm{h,t}}=4J_{\textrm{b}}+J_{\textrm{g}}+J_{\textrm{o}}=1.232\,\textrm{kgm}^{2}$. The brake torque $M_{\textrm{B,J}}$ is calculated taking the angular velocity $\omega$angular velocity into account.

$M_{\textrm{B,J}}=J_{\textrm{h,t}}\omega=J_{\textrm{h,t}}\frac{2\pi\triangle n}{\triangle t}=30.75\,\textrm{Nm}$

Theoretically, the braking torque to break down the energy input at a constant level is rising but by taking into account a with the rotational speed decreasing efficiency, this phase has been neglected. The related equation is as follows:

$M_{\textrm{B,nec}}=M_{\textrm{B,J}}+M_{\textrm{B,E}}=\frac{(\sum J)\cdot n_{\textrm{B}}}{k\cdot t_{\textrm{B}}}+\frac{P_{\textrm{input}}}{n_{\textrm{nom}}}\cdot k$

where

$P_{\textrm{input}} =$ energy input
$J =$ inertia of the system
$n_{\textrm{B}} =$ rotational speed at moment of brake activation
$n_{\textrm{nom}} =$ nominal rotational speed at moment of operation
$t_{\textrm{B}} =$ braking time
$k=\frac{60}{2\pi} =$ conversion factor

For this project an emergency braking time of about 3 seconds has been set. Furthermore, a power input at a maximum of 500 W has been assumed. The brake is activated once the overspeed limit has been triggered. The required torque to brake the rotor in 3 seconds is 42.67 Nm. This torque can be lowered by assuming higher braking times. The torque gives the range the brake should be able to cover.

### Disc Brake

To fulfill the the safety requirements to be able to react to failures in each component, a fail-safe brake system was chosen. The disc brake operates with a brake caliper activated by a spring to ensure fail-safe mode. At first we thought of buying a industrial fail-safe brake. But this would have carried some disadvantages, such as: higher costs, high energy demand (25 W, permanent), overdesigned and took to much space.

Due to that we thought of an own design for a fail-safe brake. As basis we chose a MTB disc brake that is much thiner than an industrial brake. The biggest challenge was to design a automatic actuation system that consumes the less energy possible. So we thought of a spring actuated brake. So the salient point was to find a solution to hold the spring in pretensioned position without use of electric energy. Therefore we designed a compact design in that the spring is hold by two ball-headed pressure pieces. To pretension the spring we developed a tension-lever that is connected to an brake cable of an motorcycle.

Without this developments we would not have been able to finish with the actual compact design for the wind turbine due to the short generator shaft.

Summary

The advantage of this system is that the system is independent of human control and covers every safety requirements set for this contest. A disadvantage of the fail safe brake system is the complexity it adds to the project. However, this disadvantage is compensated by the advantages in protection of the turbine operator and the turbine components itself.

Nick Hansen
I did my Bachelor of Engineering as an economical engineer with focus on energy and environmental management at the University of Applied Science Flensburg. From the start of my Bachelor I wanted to work in a field of energy efficiency or renewable energies. After finishing my Bachelor in 2013 I started the corporate Master in Wind Engineering at the UAS Kiel and UAS Flensburg.

Motivation:
I wanted to participate in this project to apply gained knowledge on field-test. With my gained knowledge in braking systems it was attractive for me to cover this field.
My tasks in this project were to think about an independent fail-safe braking system. I also took the lead in manufacturing and had an eye on the finance.

## Microcontroller, Disc Brake, Furling – How they work together

The HOLI 300 uses an Arduino microcontroller, a bike disc brake, an electrical brake and a furling sytem to limit rotor speed. In this section we explain how these components work together and how we measure rotor rotational speed. Read more

## Safety System Overview

This category is about the safety system and explains the fail-safe strategy. Read more